In designing and manufacturing a semiconductor device, there has been an increasing demand for a higher operating speed and a higher level of integration. Meanwhile, it has been pointed out that electromigration (EM) easily occurs due to an increase in current density caused by a high-speed operation or miniaturization of a wiring, so that wiring disconnection may be caused. This results in deterioration in reliability. For this reason, Cu (copper) or Ag (silver) or the like having a low resistivity has been used as a wiring material formed on a substrate of the semiconductor device. Particularly, since the cooper has a low resistivity of about 1.8 μΩ·cm and is expected to exhibit high EM resistance, it is regarded as a material suitable for achieving a high processing rate of a semiconductor device.
Typically, a damascene method has been used to form a Cu wiring on a substrate. This method includes forming a via and a trench of an insulating film by etching and filing the via and the trench of the insulating film with the Cu wiring. Further, there has been made an attempt to enhance an EM resistance of the semiconductor device by supplying a plating liquid containing CoWB (cobalt.tungsten.boron), CoWP (cobalt.tungsten.phosphorus) or the like on a surface of a substrate having a Cu wiring and by coating a metal film called as a “cap metal” on the Cu wiring by means of electroless plating (see, for example, Patent Document 1).
The cap metal is formed by supplying the electroless plating liquid on the surface of the substrate having the Cu wiring. By way of example, the substrate is held on a rotatable holder, and the electroless plating liquid is supplied while rotating the rotatable holder, so that a uniform liquid flow can be formed on the surface of the substrate. Thus, a uniform cap metal can be formed over the entire surface of the substrate (see, for example, Patent Document 2).
However, as for the electroless plating, it has been known that a precipitation ratio of metal is highly affected by reaction conditions such as a composition and a temperature of the plating liquid. Further, it has been known that particles are generated from the plating liquid heated to a processing temperature as a result of the reaction. For this reason, in a process for continuously performing plating processes on a wafer, if the process is stopped due to a request or a trouble in the process, a quality of the plating liquid heated to the processing temperature is deteriorated. Therefore, it is difficult to form a cap metal having a uniform film thickness on the wafer.    Patent Document 1: Japan Patent Laid-open Publication No. 2006-111938    Patent Document 2: Japan Patent Laid-open Publication No. 2001-073157